This invention relates to an ink for heat-sensitive recording with which a substrate such as a plastic sheet can be partially printed and has strong adhesion to the substrate and forms a coat layer having high water-resistance.
Generally, heat-sensitive recording materials comprise a support having provided thereon a heat-sensitive recording layer comprising as essential components an electron donating, colorless dye precursor and an electron accepting color developer. Upon heating the heat-sensitive recording material by a thermal head, a thermal pen, a laser beam or the like, the dye precursor and the color developer instantly react with each other to give recorded images. Such heat-sensitive recording materials are disclosed in Japanese Patent Application Kokoku Nos. 43-4160, 45-14039 and the like.
If such heat-sensitive recording materials are used, records can be obtained by relatively simple apparatus, the maintenance of the apparatus is easy and no noise is made. Thus, the heat-sensitive recording materials are used in a wide variety of fields such as measuring recorders; facsimiles; printers; terminals of computers; automatic vending machines for labels, tickets and the like; etc.
Generally, these widely used heat-sensitive recording materials are obtained by coating the whole surface of paper used as a support with an aqueous coating composition for heat-sensitive recording. Recently, however, as another kind of heat-sensitive recording material, there are widely used prepaid cards such as shopping cards, highway cards and cards used in traffic facilities such as railroads, buses, taxies and the like. In manufacturing these cards, it is not necessary to coat the whole surface of the substrate with a coating composition for heat-sensitive recording, and in many cases, a partial printing is conducted in which only a given portion of the surface of the substrate is printed.
The heat-sensitive recording materials in which an electron donating, colorless dye precursor and an electron accepting color developer are used are advantageous in that they have good appearance and soft feel, the color density obtained is high and various hues can be obtained. However, these materials have poor record preservability for the following reasons: When the recorded area comes into contact with plastics such as polyvinylchloride or the like, the records disappear on account of a plasticizer, an additive or the like contained in the plastics. When the recorded area comes into contact with an agent contained in a food or a cosmetic, the records disappear. The records are extinguished by sunlight exposure in a short time. At present, therefore, the use of the above heat-sensitive recording materials is limited to some fields. Thus, there has been strongly desired development of a heat-sensitive recording material free from these disadvantages.
As heat-sensitive recording materials which give recorded images having high preservability by heating the two components contained therein to react with each other, there are disclosed heat-sensitive recording materials wherein the two components are an imino-compound and an isocyanate compound in, for example, Japanese Application Kokai Nos. 58-38733, 58-54085, 58-104959, 58-149388, 58-115887, 59-115888 and U.S. Pat. No. 4,521,793.
Since the heat-sensitive recording materials comprising an isocyanate compound and an imino compound are excellent in preservability of recorded images, there are many literature references relating to the technique of coating the whole surface of a paper substrate with an aqueous coating composition of the two components.
However, there are many problems in partial printing by a printing machine or the like. For example, partial printing with the aqueous coating composition requires a long time in the drying step. And, a plastic substrate is inferior in wettability by an aqueous coating composition to the paper substrate. Therefore, in coating the plastic substrate with an aqueous coating composition, it is impossible to obtain a uniform coating because beading phenomenon appears. Moreover, the coat layer obtained thereby has poor adhesion to the substrate and may be easily peeled from the substrate when it is wetted by water and then rubbed.